Thin film actuated mirror array and method for the manufacture thereof

ABSTRACT

There is provided an array of M×N thin film actuated mirrors for use in an optical projection system comprising an active matrix, an array of M×N thin film actuating structures, each of the thin film actuating structures including at least a thin film layer of a motion-inducing material, a pair of electrodes, each of the electrodes being provided on top and bottom of the thin film motion-inducing layer, an array of M×N supporting members, each of the supporting members being used for holding each of the actuating structures in place by cantilevering each of the actuating structures and also for electrically connecting each of the actuating structures and the active matrix, and an array of M×N mirrors for reflecting light beams, each of the mirrors being placed on top of each of the actuating structures. An electrical signal is applied across the thin film layer of the motion-inducing material located between the pair of electrodes in each of the actuating structures, causing a deformation thereof, which will in turn deform the mirror placed on top thereof.

FIELD OF THE INVENTION

The present invention relates to an optical projection system; and, moreparticularly, to an array of M×N thin film actuated mirrors for use inthe system.

BACKGROUND OF THE INVENTION

Among the various video display systems available in the art, an opticalprojection system is known to be capable of providing a high qualitydisplay in a large scale. In such an optical projection system, lightfrom a lamp is uniformly illuminated onto an array of, e.g., M×N,actuated mirrors such that each of the mirrors is coupled with each ofthe actuators. The actuators may be made of an electrodisplacivematerial such as a piezoelectric or an electrostrictive material whichdeforms in response to an electric field applied thereto.

The reflected light beam from each of the mirrors is incident upon anaperture of a baffle. By applying an electrical signal to each of theactuators, the relative position of each of the mirrors to the incidentlight beam is altered, thereby causing a deviation in the optical pathof the reflected beam from each of the mirrors. As the optical path ofeach of the reflected beams is varied, the amount of light reflectedfrom each of the mirrors which passes through the aperture is changed,thereby modulating the intensity of the beam. The modulated beamsthrough the aperture are transmitted onto a projection screen via anappropriate optical device such as a projection lens, to thereby displayan image thereon.

In FIG. 1, there is shown a cross sectional view of an M×Nelectrodisplacive actuated mirror array 10 for use in an opticalprojection system, disclosed in a copending commonly owned application,U.S. Ser. No. 08/278,472, entitled "ELECTRODISPLACIVE ACTUATED MIRRORARRAY", comprising: an active matrix 11 including a substrate 12 and anarray of M×N transistors thereon; an array 13 of M×N electrodisplaciveactuators 30, each of the electrodisplacive actuators 30 including apair of actuating members 14, 15, a pair of bias electrodes 16, 17, anda common signal electrode 18; an array 19 of M×N hinges 31, each of thehinges 31 fitted in each of the electrodisplacive actuators 30; an array20 of M×N connecting terminals 22, each of the connecting terminals 22being used for electrically connecting each of the signal electrodes 18with the active matrix 11; and an array 21 of M×N mirrors 23, each ofthe mirrors 23 being mounted on top of each of the M×N hinges 31.

In the above mentioned copending, commonly owned application, there isalso disclosed a method for manufacturing such an array of M×Nelectrodisplacive actuated mirrors, employing a ceramic wafer having athickness of 30 to 50 μm.

There is room for further improvements over the above described methodfor manufacturing an array of M×N electrodisplacive actuators, however.First of all, it is rather difficult to obtain a ceramic wafer having athickness of 30 to 50 μm; and, furthermore, once the thickness of theceramic wafer is reduced to a 30 to 50 μm range, the mechanicalproperties thereof are likely to degrade which may, in turn, make itdifficult to carry out the manufacturing process.

In addition, it involves a number of time consuming, hard to control,and tedious processes, thereby making it difficult to obtain the desiredreproducibility, reliability and yield; and, furthermore, there may be alimit to the down sizing thereof.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention is toprovide a method for manufacturing an array of M×N actuated mirrors,which dispenses with the use of a thin electrodisplacive ceramic wafer.

It is another object of the present invention to provide an improved andnovel method for manufacturing an array of M×N actuated mirrors whichwill give higher reproducibility, reliability and yield by utilizing theknown thin film techniques commonly employed in the manufacture ofsemiconductors.

It is a further object of the present invention to provide an array ofM×N actuated mirrors having a novel structure, incorporating a pluralityof thin film layers of a motion-inducing, an electrically conducting anda light reflecting materials.

In accordance with one aspect of the present invention, there isprovided an array of M×N thin film actuated mirrors for use in anoptical projection system, the array comprising: an active matrixincluding a substrate, an array of M×N transistors and an array of M×Nconnecting terminals; an array of M×N thin film actuating structures,each of the actuating structures being provided with a top and a bottomsurfaces, a proximal and a distal ends, each of the actuating structuresincluding at least a thin film layer of a motion-inducing materialhaving a top and bottom surfaces, and a first and a second electrodes ofa specific thickness, the first electrode being placed on the topsurface of the motion-inducing layer and the second electrode, on thebottom surface thereof, wherein an electrical signal applied across themotion-inducing layer between the first and second electrodes causes adeformation of the motion-inducing layer, and hence the actuatingstructure; an array of M×N supporting members, each of the supportingmembers being provided with a top and a bottom surfaces, wherein each ofthe supporting members is used for holding each of the actuatingstructures in place and also electrically connecting each of theactuating structures and the active matrix; and an array of M×N mirrorsfor reflecting light beams, wherein each of the mirrors is placed on topof each of the actuating structures such that each of the mirrorsdeforms in response to the deformation of each of the actuatingstructures.

In accordance with another aspect of the present invention, there isprovided a novel method for manufacturing an array of M×N actuatedmirrors for use in an optical projection system, utilizing the knownthin film techniques, the method comprising the steps of: (a) providingan active matrix having a top and a bottom surfaces, the active matrixincluding a substrate, an array of M×N transistors and an array of M×Nconnecting terminals; (b) forming a supporting layer on the top surfaceof the active matrix, the supporting layer having an array of M×Npedestals corresponding to the array of M×N supporting members in thearray of M×N thin film actuated mirrors and a sacrificial area; (c)treating the sacrificial area of the supporting layer to be removable;(d) depositing a first thin film electrode layer on the supportinglayer; (e) providing a thin film motion-inducing layer on the first thinfilm electrode layer; (f) forming a second thin film electrode layer onthe thin film motion-inducing layer; (g) depositing a mirror layer, madeof a light reflecting material, on the second thin film electrode layer;and (h) removing the sacrificial area of the supporting layer to therebyform said array of M×N thin film actuated mirrors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a cross sectional view of an array of M×N electrodisplaciveactuated mirrors previously disclosed;

FIG. 2 represents a cross sectional view of an array of M×N thin filmactuated mirrors is accordance with a preferred embodiment of thepresent invention;

FIG. 3 illustrates a detailed cross sectional view of an thin filmactuated mirror of the first embodiment shown in FIG. 2;

FIG. 4 offers a cross sectional view of an actuated mirror of the firstembodiment with an elastic layer added intermediate the mirror and thefirst electrode;

FIG. 5 depicts a cross sectional view of an actuated mirror of the firstembodiment with an elastic layer placed on bottom of the secondelectrode;

FIG. 6 presents a cross sectional view of an actuated mirror of thefirst embodiment having the first electrode made of a light reflectingmaterial and provided with the first and second electrodes having adifferent thickness;

FIG. 7 describes a cross sectional view of an actuated mirror of thefirst embodiment having the first electrode made of a light reflectingmaterial and provided with an elastic layer placed on the bottom surfaceof the actuating structure;

FIG. 8 explains a cross sectional view of an actuated mirror of thefirst embodiment with an elastic layer placed on top of the firstelectrode and made of a light reflecting material;

FIGS. 9A and 9B demonstrate a cross sectional view of an actuated mirrorof the first embodiment having either one of the top and bottom surfacesof the motion-inducing layer in each of the actuating structure coveredpartially with the first and second electrodes;

FIG. 10 discloses a cross sectional view of an actuated mirror of thefirst embodiment in an actuated state;

FIG. 11 provides a cross sectional view of an actuated mirror of thesecond embodiment having a bimorph structure;

FIG. 12 displays a cross sectional view of an actuated mirror of thesecond embodiment having the first electrode made of a light reflectingmaterial; and

FIGS. 13A to 13F reproduce schematic cross sectional views setting forththe manufacturing steps for the first embodiment in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 2 to 13, there are provided schematic crosssectional views of the inventive array of M×N thin film actuated mirrorsfor use in an optical projection system, wherein M and N are integers,in accordance with preferred embodiments of the present invention. Itshould be noted that like parts appearing in FIGS. 2 to 13 arerepresented by like reference numerals.

In FIG. 2, there is illustrated a cross sectional view of a firstembodiment of an array 50 of M×N thin film actuated mirrors 51,comprising an active matrix 52, an array 53 of M×N thin film actuatingstructures 54, an array 55 of M×N supporting members 56 and an array 57of M×N mirrors 58.

FIG. 3 represents a detailed cross sectional view of a thin filmactuated mirror 51 shown in FIG. 2. The active matrix 52 includes asubstrate 59, an array of M×N transistors (not shown) and an array 60 ofM×N connecting terminals 61. Each of the thin film actuating structures54 is provided with a top and a bottom surfaces 61, 63, a proximal and adistal ends, 64, 65, and further includes at least a thin film layer 66of a motion inducing material having a top and a bottom surfaces 67, 68and a first and second electrodes 69, 70 of a specific thickness, madeof, e.g., a metal such as gold (Au) or silver (Ag), the first electrode69 having a top surface 39. The first electrode 69 is placed on the topsurface 67 of the motion-inducing thin film layer 66 and the secondelectrode 70, on the bottom surface 68 thereof. The motion-inducing thinfilm layer 66 is made of a piezoelectric ceramic, an electrostrictiveceramic, a magnetrostrictive ceramic or a piezoelectric polymer. In thecase when the motion-inducing thin film layer is made of a piezoelectricceramic or a piezoelectric polymer, it must be poled.

Each of the M×N supporting members 56, provided with a top and bottomsurfaces 71, 72, is used for holding each of the actuating structures 54in place and also for electrically connecting the second electrode 70 ineach of the actuating structures 54 with the corresponding connectingterminals 61 on the active matrix 52 by being provided with a conduit 73made of an electrically conductive material, e.g., a metal. In thisinventive array 50 of M×N thin film actuated mirrors 51, each of theactuating structures 54 is cantilevered from each of the supportingmembers 56 by being mounted on the top surface 71 of each of thesupporting members 56 at the bottom surface 63 of each of the actuatingstructures 54 at the proximal end 64 thereof, and the bottom surface 72of each of the supporting members 56 is placed on top of the activematrix 52. Each of the M×N mirrors 58 for reflecting light beams isplaced on top of each of the actuating structures 54.

An electrical field is applied across the motion-inducing thin filmlayer 66 between the first and second electrodes 69, 70 in each of theactuating structures 54. The application of such an electric field willcause a deformation of the motion-inducing layer 66, hence the actuatingstructure 54, and hence the mirror 58 placed on top thereof.

In order for the array 50 of M×N thin film actuated mirrors 51 tofunction properly, the combined thickness of the mirror 58 and the firstelectrode 69 in each of the thin film actuated mirrors 51 must bedifferent from that of the second electrode therein for the deformationthereof to take place. If not, an elastic layer 88 having a top surface40 must be further provided to each of the actuated mirrors 51. Theelastic layer 88 can be placed either intermediate the mirror 58 and thefirst electrode 69 or on bottom of the second electrode 70 in each ofthe actuated mirrors 51, as illustrated in FIGS. 4 and 5.

The electrically conducting material making up the first electrode 69can be also light reflecting, e.g., aluminum (Al), which will allow thetop surface 39 of the first electrode function also as the mirror 58 ineach of the thin film actuated mirrors 51. In such a case, in order foreach of the thin film actuated mirrors 51 to function properly, thefirst and the second electrodes 69, 70 must be of a different thicknessor each of the thin film actuated mirrors 51 must be provided with anelastic layer 88 placed on the bottom surface of each of the actuatingstructures therein, as shown in FIGS. 6 and 7. Furthermore, if theelastic layer 88 is made of a light reflecting material, it can alsofunction as the mirror 58, as depicted in FIG. 8.

It is possible for the inventive array 50 of thin film actuated mirrors51 to function equally well by having the top and bottom surfaces 67, 68of the motion-inducing thin film layer 66 in each of the actuatingstructures 54 covered completely with the first and second electrodes69, 70 or by having either one of the top and bottom surfaces 69, 70 ofthe motion-inducing thin film layer 66 in each of the actuatingstructures 54 covered partially with the first and second electrodes 69,70. Two examples of the actuated mirror 51 having such a structure areillustrated in FIGS. 9A and 9B.

By way of example of the first embodiment, there are illustrated isFIGS. 8 and 10 an array 50 of M×N thin film actuated mirrors 51comprising an array of M×N actuating structures 54, made of apiezoelectric ceramic, e.g., lead zirconium titanate(PZT). An electricfield is applied across the motion-inducing thin film piezoelectriclayer 66 located between the first and second electrodes 69, 70 in eachof the actuating structures 54. The application of the electric fieldwill either cause the piezoelectric ceramic to contract or expand,depending on the polarity of the electric field with respect to thepoling of the piezoelectric material. If the polarity of the electricfield corresponds to the polarity of the piezoelectric corresponds tothe polarity of the piezoelectric ceramic, the piezoelectric ceramicwill contract. If the polarity of the electric field is opposite thepolarity of the piezoelectric ceramic, the piezoelectric ceramic willexpand.

With reference to FIGS. 8 and 10 the polarity of the piezoelectricceramic corresponds to the polarity of the applied electric field,causing the piezoelectric ceramic to contract.

Because the elastic layer 88 does not contract, the actuating structurebends downward, as shown in FIG. 10. Referring now to FIGS. 8 and 10, itcan be shown that the light impinging the top surface 40 of the elasticlayer 88, functioning as the mirror 58, of the actuated mirror 51 shownin FIG. 10 is deflected at a larger angle than the light reflected fromof the unactuated actuated mirror 51 shown in FIG. 8.

Alternatively, an electric field of a reverse polarity may be appliedacross the motion-inducing thin film piezoelectric layer 66, causing thepiezoelectric ceramic to expand. In this example, the elastic layer 88does not expand, and as a result, the actuating structure 54 bendsupward (not shown). The light impinging the mirror 58 of the upwardlyactuated mirror 51 is deflected at a smaller angle than the lightdeflected from the top surface 40 of the elastic layer 88 of theunactuated actuated mirror 51 shown in FIG. 8.

There is shown in FIG. 11, a cross sectional view of a second embodimentof an array 100 of M×N thin film actuated mirrors 101, wherein thesecond embodiment is similar to the first embodiment except that each ofthe actuating structures 54 is of a bimorph structure, including a firstelectrode 69, a second electrode 70, an intermediate metal layers 87 anupper motion-inducing thin film layer 89 having a top and a bottomsurfaces 90, 91 and a lower motion-inducing thin film layer 92 providedwith a top and bottom surfaces 93, 94. In each of the actuatingstructures 54, the upper and lower motion-inducing thin film layers89,92 are separated by the intermediate metal layer 87, the firstelectrode 69 placed on the top surface 90 of the upper motion-inducingthin film layer 89, and the second electrode 70, on the bottom surface94 of the lower motion-inducing thin film layer 92.

As in the case of the first embodiment, the upper and lowermotion-inducing thin film layers 89, 92 in each of the actuatingstructures 54 are made of a piezoelectric ceramic, an electrostrictiveceramic, a magnetostrictive ceramic or a piezoelectric polymer. In thecase when the upper and lower motion-inducing thin film layers 89,92 aremade of a piezoelectric ceramic or a piezoelectric polymer, the upperand lower motion-inducing thin film layers 89,92 must be poled in such away that the polarization direction of piezoelectric material in theupper motion-inducing thin film layer 89 is opposite from that of thelower motion-inducing thin film layer 92. FIG. 12 illustrates a crosssectional view of an actuated mirror 101 of the second embodiment,wherein the first electrode 69 is made of a light reflecting material,thereby allowing the top surface 39 thereof to also function as themirror 58.

As an example of how the second embodiment functions, assume that theupper and lower motion-inducing layers 89, 90 in the array 100 of M×Nthin film actuated mirrors 101 shown in FIG. 11 are made of apiezoelectric ceramic, e.g., PZT. When an electric field is appliedacross each of the actuating structure 54, the upper and lowermotion-inducing thin film piezoelectric layers 89, 92, the actuatingstructure 54 will either bend upward or downward, depending on thepoling of the piezoelectric ceramic and the polarity of the electricfield. For example, of the polarity causes the upper motion-inducingthin film piezoelectric layer 89 to contract, and the lowermotion-inducing thin film piezoelectric layer 92 to expand, theactuating structure 54 will bend upward. In this situation, theimpinging light is deflected of a smaller angle from the actuatingstructure 54 than the deflected light from the unactuated actuatingstructure 54. However if the polarity of the piezoelectric ceramic andthe electric field causes the upper motion-inducing thin filmpiezoelectric layer 89 to expand and the lower motion-inducing thin filmpiezoelectric layer 92 to contract, the actuating structure 54 will benddownward. In this situation, the impinging light is deflected at alarger angle from the actuating structure 54 than the deflected lightfrom the unactuated actuating structure 54.

There are illustrated in FIGS. 13A to 13F manufacturing steps involvedin manufacturing of the first embodiment of the present invention. Theprocess for manufacturing the first embodiment, i.e., the array 50 ofM×N thin film actuated mirror 51, wherein M×N are integers, begins withthe preparation of the active matrix 52, having a top and a bottomsurfaces 75, 76, comprising the substrate 59, the array of M×Ntransistors (not shown) and the array 60 of M×N connecting terminals 61,as illustrated in FIG. 13A.

In the subsequent step, there is formed on the top surface 75 of theactive matrix 52 a supporting layer 80, including an array 81 of M×Npedestals 82 corresponding to the array 55 of M×N supporting members 56and a sacrificial area 83, wherein the supporting layer 80 is formed by:depositing a sacrificial layer (not shown) on the entirety of the topsurface 75 of the active matrix 52; forming an array of M×N empty slots(not shown), to thereby generated the sacrificial area 83, each of theempty slots being located around each of the M×N connecting terminals61; and providing a pedestal 82 in each of the empty slots, as shown inFIG. 13B. The sacrificial layer is formed by using a sputtering method,the array of empty slots, using an etching method, and the pedestals,using a sputtering or a chemical vapor deposition (CVD) method, followedby an etching method. The sacrificial area 83 of the supporting layer 80is then treated so as to be removable later using an etching method orthe application of chemicals.

A conduit 73 for electrically connecting each of the connectingterminals 61 with each of the second electrode 70, made of anelectrically conductive material, e.g., tungsten(W), is formed in eachof the pedestals 82 by first creating a hole extending from top thereofto top of the corresponding connecting terminals 61 using an etchingmethod, followed by filling therein with the electrically conductingmaterial, as depicted in FIG. 13(C).

In the subsequent step, as depicted in FIG. 13D, a first thin filmelectrode layer 84, made of an electrically conducting material, e.g.,Au, is deposited on the supporting layer 80. Thereafter, a thin filmmotion-inducing layer 85, made of a motion-inducing material, e.g., PZT,and a second thin film electrode layer 95 are then respectively formedon the first thin film electrode layer 84.

Subsequently, a thin film layer 99 of a light reflecting material, e.g.,Al, is provided on top of the second electrode layer 95.

The thin film layers of the electrically conducting, themotion-inducing, and the light reflecting materials may be deposited andpatterned with the known thin film techniques, such as sputtering,sol-gel, evaporation, etching and micro-machining, as shown in FIG. 13E.

The sacrificial area 83 of the supporting layer 80 is then removed ordissolved by the application of chemical to thereby form said array 50of M×N thin film actuated mirrors 51, as illustrated in FIG. 13F.

The second embodiment is fabricated in a similar manner as the firstembodiment. The supporting layer is applied to the active matrix. Thesupporting layer also includes the array of M×N pedestals correspondingto the array of M×N supporting members and the sacrificial area. Thefirst thin film electrode layer, the lower thin film motion-inducinglayer, the intermediate metal layer, the upper thin film motion-inducinglayer, the second thin film electrode layer, and the light reflectinglayer are then deposited respectively on the supporting layer. The thinfilm layers of an electrically conducting, a motion-inducing and a lightreflecting materials may be deposited and patterned with the known thinfilm techniques, as stated earlier. The sacrificial area of thesupporting layer is next dissolved or removed by the application of achemical, leaving the array 100 of thin film actuated mirrors 101,having the array 53 of M×N actuating structures 54 with the bimorphstructure, each of the actuating structures 54 being cantilevered fromeach of the supporting members 56.

In the above described methods for manufacturing the first and secondembodiments of the present invention, an additional process for formingthe elastic layer 88 can be added, involving a similar process as in theforming of other thin film layers.

While the present invention has been described with respect to certainpreferred embodiments only, other modifications and variations may bemade without departing from the scope of the present invention as setforth in the following claims.

What is claimed is:
 1. An array of M×N thin film actuated mirrors,wherein M and N are integers, for use in an optical projection system,comprising:an active matrix including a substrate and an array of M×Nconnecting terminals; an array of M×N thin film actuating structures,each of the actuating structures being provided with a top and a bottomsurfaces, a proximal and a distal ends, each of the actuating structuresincluding at least a thin film layer of a motion-inducing materialhaving a top and bottom surfaces, and a first and a second electrodes ofa specific thickness, the first electrode being placed on the topsurface of the motion-inducing thin film layer and the second electrode,on the bottom surface thereof, wherein an electrical signal appliedacross the motion-inducing thin film layer between the first and secondelectrodes causes a deformation of the motion-inducing layer, and theactuating structures; an array of M×N supporting members, each of thesupporting members being provided with a top and a bottom surfaces,wherein each of the supporting members is used for holding each of theactuating structures in place and also electrically connecting each ofthe actuating structures with the active matrix, wherein each of theactuating structures is cantilevered from each of the supporting membersby being mounted on the top surface of each of the supporting members atthe bottom surface of each of the actuating structures at the proximalend; and an array of M×N mirrors for reflecting light beams, whereineach of the mirrors is placed on top of each of the actuating structuressuch that each of the mirrors deforms in response to the deformation ofeach of the actuating structures.
 2. The actuated mirror array of claim1, wherein the bottom surface of each of the supporting members isplaced on top of the active matrix.
 3. The actuated mirror array ofclaim 1, wherein the motion-inducing thin film layer is made of apiezoelectric ceramic or a piezoelectric polymer.
 4. The actuated mirrorarray of claim 3, wherein the motion-inducing thin film layer is poled.5. The actuated mirror array of claim 1, wherein the motion-inducingthin film layer is made of an electrostrictive material.
 6. The actuatedmirror array of claim 1, wherein the motion-inducing thin film layer ismade of a magnetostrictive material.
 7. The actuated mirror array ofclaim 1, wherein each of the supporting members is provided with aconduit for electrically connecting the second electrode in each of theactuating structures with the corresponding connecting terminal on theactive matrix.
 8. The actuated mirror array of claim 1, wherein each ofthe M×N mirrors is made of a light reflecting material.
 9. The actuatedmirror array of claim 1, wherein the first and second electrodes covercompletely the top and bottom surfaces of the motion-inducing thin filmlayer, respectively.
 10. The actuated mirror array of claim 1, whereineither the first or the second electrode covers partially the top or thebottom surface of the motion-inducing thin film layer.
 11. The actuatedmirror array of claim 1, wherein the first and second electrodes aremade of an electrically conducting material.
 12. The actuated mirrorarray of claim 1, further comprising M×N elastic layers, each of theelastic layers being placed on the top surface of each of the actuatingstructures.
 13. The actuated mirror array of claim 12, wherein each ofthe elastic layers is disposed between the mirror and the firstelectrode in each of the actuating structures.
 14. The actuated mirrorarray of claim 1, further comprising M×N elastic layers, each of theelastic layers being placed on the bottom surface of each of theactuating structures.
 15. The actuated mirror array of claim 11, whereinthe first electrode is made of a light reflecting material, to therebyallowing the first electrode to function also as the mirror in each ofthe thin film actuated mirrors.
 16. The actuated mirror array of claim15, wherein the first electrode has the same thickness as the secondelectrode in each of the actuated mirrors.
 17. The actuated, mirrorarray of claim 18, further comprising M×N elastic layers, each of theelastic layers being placed on the bottom surface of each of theactuating structures.
 18. The actuated mirror array of claim 15, whereinthe first electrode has a different thickness from the second electrodein each of the actuating structures.